It is known to determine the position of a mobile communications device (often referred to in the art as “user equipment”) by observing signals that are available in the local environment and are detectable by a receiver in the device. These signals are not necessarily intended for providing a positioning function and are sometimes termed “Signals of Opportunity” (“SoOps”).
In particular, it is known to use the signals transmitted by cellular base stations in a cellular communications network as SoOps, for calculating position. Base station signals are advantageous from this point of view, because the signal from each individual base station is transmitted over a relatively small area. Additionally, base station signals tend to have relatively stable timing, because communications in all cellular networks rely, to a greater or lesser extent, on having a consistent timing reference and frequency reference.
One approach is for the device to measure the time of arrival of a defined portion of the signal from a given base station. This procedure is then repeated for other signals, from other base stations that are “visible” from the device's position. Each time of arrival can be used to calculate a ranging measurement between the device and the respective base station. With enough of these ranging measurements, and some additional knowledge (for example, about the locations of the base stations), it is possible to calculate the position of the device, by trilateration. The process is analogous to that used by satellite positioning receivers to calculate their positions using satellite positioning signals, but with the difference that the cellular signals are not primarily intended for positioning applications.
Using the time of arrival (TOA) of the base station signal to calculate a ranging measurement relies on the assumption that the signal arrived at the mobile communications device from the base station via a direct, line-of-sight propagation path. And since the accuracy of the position calculation depends on measuring the time of arrival accurately, anything that interferes with the time of arrival measurement will degrade the accuracy of the resulting position estimate. Multipath conditions pose a particular problem, because they can lead to ambiguity in the measured time of arrival. But multipath conditions are widely prevalent—especially in dense urban environments, where each building may be capable of reflecting the base station signal and creating an additional multipath component.
There is therefore a need for approaches that are more robust to multipath effects.